1. Field of the Invention
The present invention relates to a retardation element, a liquid crystal display device, and a projection display device.
2. Description of the Related Art
In recent years, an optical compensation technique utilizing a retardation element has been applied to projection display devices in order to improve a contrast property and a viewing angle property. Examples include black luminance correction in a vertically aligned liquid crystal. In order to correct polarization disturbance due to a pretilt angle of a liquid crystal, birefringence of oblique incident light, or the like, there are considered a method of performing optical compensation by placing a retardation element made of crystal or the like in parallel with a surface of a liquid crystal panel, and a method of performing optical compensation by placing an organic material having a birefringence such as a polymer film in parallel with a surface of a liquid crystal panel (see Japanese Patent Application Laid-Open (JP-A) Nos. 2005-172984 and 2007-101764, and Japanese Patent (JP-B) No. 4566275).
However, in a method of machining a monocrystal as a retardation element, in particular, with a view to performing compensation taking into account even a pretilt angle of a liquid crystal, it is necessary to cut out the monocrystal at a predetermined angle from the crystal axis. This necessitates a very high level precision in cutting, polishing, etc. of the material, and high costs are required to realize such a precision. Furthermore, axis control is not easy with a stretched film or the like.
Hence, there is considered a method of placing a retardation element itself to be inclined from a liquid crystal panel (see JP-A No. 2009-229804). However, there is a risk of shortage of space needed for the inclination in projectors that are becoming increasingly smaller in size. Furthermore, there is a problem in durability, with susceptibility to deterioration due to heat and UV rays.
Further, in projection display devices, a ½ wave plate is used as part of a polarization conversion element. For example, there is proposed a technique of cutting a crystal for a ½ wave plate from an angle such that an optical axis of the wave plate may not be parallel with a substrate surface but may be inclined therefrom, and sticking two such inclined crystal wave plates together, to thereby widen a wavelength range and suppress incident angle dependency (see JP-A Nos. 2004-354935, 2009-133917, and 2012-078436). However, a crystal substrate that is expensive may require a greater expense when it needs to be cut out to have an inclined optical axis.
As described above, there have been increasing needs for inclining the optical axis from a direction normal to a substrate or from an in-plane direction, but there have been technical and cost problems.
Meanwhile, an obliquely deposited retardation element excellent in light resistance and heat resistance is an element that is, in the first place, produced to have an inclined optical axis. However, the problem is the directional limitation of the optical axis. A detailed explanation will be provided below. A common oblique deposition procedure can make deposition particles incident to the substrate from an oblique direction and produce film density anisotropy by a so-called self-shadowing effect. The density anisotropy can produce anisotropy in the refractive index of the film in the in-plane direction of the substrate, which forms a factor for birefringence to occur. Here, it is possible to impart a relatively arbitrary retardation to light that is incident from a direction normal to the substrate, based on the film thickness. In oblique deposition, the growth angle of deposition particles is determined by the angle of the oblique deposition, and the optical axis is determined by the direction of the deposition particles. In order to change the optical axis, i.e., the growth direction of the deposition particles, it is necessary to change the incident angle of the oblique deposition, whereas when the incident angle is reduced, birefringence is reduced. Therefore, there is a problem of difficulty producing a retardation element having an optical axis which has a small inclination angle (i.e., which is close to the direction normal to the substrate) (see APPLIED OPTICS/Vol. 28, No. 13/1 July 1989).
There is also proposed a technique of alternately performing a step of performing oblique deposition while rotating a substrate, and a step of performing oblique deposition from a specific direction while stopping the rotation (see U.S. Pat. No. 6,206,065). However, this proposed technique needs to perform oblique deposition while rotating a substrate. Therefore, there is a problem that the production apparatus is complicated.
Hence, it is currently requested to provide a retardation element that can be produced with a simple production apparatus, of which optical axis and retardation are adjustable easily, and that is excellent in heat resistance and durability, and a liquid crystal display device, and a projection display device